Packet switched network protocols, in which data is sent from one node to another in the form of one or more discrete packets, have long been used in conventional wired networks to manage the exchange of data between network nodes. In wireless communications, it is sometimes advantageous to employ a protocol that allows for the transmission and receipt of data packets. One such environment is the wireless local area network (LAN) environment.
To facilitate the development and use of wireless LAN (“WLAN” or “WiLAN”) technology, the Institute of Electrical and Electronics Engineers (IEEE) has promulgated a series of standards, the IEEE Standard 802.11 series, to provide standardized specifications for the medium access control (MAC) and physical layer (PHY) for such a wireless LAN. IEEE Standard 802.11b, for example, provides the specifications for extending the PHY layer in the 2.45 GHz band.
Like packet switched communication protocols in a wired network environment, the IEEE Standard 802.11b prescribes a packet format in which each packet comprises a preamble portion, a header portion, and a data portion, transmitted in that order. In general, the preamble portion is used by the receiving system to recognize that a packet is being received and to perform synchronization operations to enable the receiving system to reliably receive and interpret the incoming packet. The header portion typically is used to provide to the receiving system information concerning the data portion that will follow, such as by indicating how much data is being sent in the packet and what signaling or protocol is being used to transmit the data portion. The data portion, sometimes referred to as the “payload” of the packet, comprises all or a part of a message the sending system is communicating to the receiving system. The message may comprise, for example, a request for data available from the receiving system or data sent by the sending system in response to a request received from the receiving system.
In certain environments, such as in a wireless LAN environment operating under IEEE Standard 802.11b, more than one packet format may be prescribed. For example, IEEE Standard 802.11b specifies a long preamble format and a short preamble format. The long preamble format may be needed, for example, for use with legacy or other systems that employ the 1 Megabit per second (Mbps) mode of operation under IEEE Standard 802.11b. The short preamble format, on the other hand, may be suitable for use with higher data rate modes of operation, such as for 2 Mbps, 5.5 Mbps, and 11 Mbps modes of operation.
Under IEEE Standard 802.11b, for both the long preamble format and the short preamble format the preamble comprises a first set of bits to be used for synchronization operation and a second set of bits, termed the “start frame delimiter” (SFD), which second set of bits the receiving system uses to recognize that the end of the preamble has been reached and the header portion of the packet is about to begin. Under IEEE Standard 802.11b, the long preamble comprises 128 bits for synchronization and a 16 bit SFD, whereas the short preamble comprises 56 bits for synchronization and a different 16 bit SFD. Under IEEE Standard 802.11b, the SFD is a prescribed 16-bit sequence, with the short preamble SFD being the reverse of the long preamble SFD. A receiving system typically looks at the decoded, de-scrambled received signal to determine when a match for the SFD corresponding to the preamble format used for the packet has been received.
In general, the more received bits the receiving system has available to it for synchronization, the better, because by performing synchronization operations using more received bits the receiving system can achieve better synchronization with the transmitting system and better channel sensitivity, with the result that the data packet is received and interpreted more reliably. For example, in a system in which channel estimation is used to create a model of the channel and correct at the receiving end for any changes made to the transmitted signal by the channel before the signal is received by the receiving system, be able to devote more bits to channel estimation typically would result in a more accurate and/or complete model of the channel.
In certain implementations, such as where synchronization is performed using the decoded but not yet de-scrambled received signal, it would be necessary to consume one or more of the bits intended for synchronization in the initialization of a de-scrambler at the receiving system in order to detect the SFD. It would be advantageous to not lose the availability of these bits for synchronization operations. In addition, especially where a short preamble is used, it would be advantageous to make more bits of the preamble available for synchronization while still reliably detecting the SFD.